Such a communication system is known that, when an instruction/correspondence with data is transmitted to a plurality of mobile stations from a base station, a mobile station number and reply order data of each mobile station are attached to the instruction/correspondence data, and the instruction/correspondence data is transmitted to the plurality of mobile stations. Each mobile station having received the instruction/correspondence data replies with a confirmation signal according to the reply order. Thereby, data collision is avoided.    Patent Document 1: Japanese Laid-Open Patent Application No. 2004-166134    Non-patent Document 1: www.mlit.go.jp/jidosha/anzen/asv/index.html Oct. 26, 2006